Lithium ion secondary battery is generally formed by disposing an electrolyte between a positive electrode and a negative electrode. The positive electrode and negative electrode are respectively composed by forming a layer containing an active material, a conductive material, a binder and the like on the surface of a collector. As the active material for the positive electrode, Li—Mn type composite oxide, Li—Ni type composite oxide, Li—Co type composite oxide and the like are used and as the active material for the negative electrode, carbon materials are used.
Since lithium ion secondary batteries are capable of achieving high energy density and high voltage as compared to Nickel-Cadmium batteries and the like, they have been rapidly employed in recent years as an operate source of portable equipment, such as portable telephones and notebook personal computers. An expansion in the applicable range is expected in the future. In view of this, there have been actively studied lithium ion secondary batteries aiming for improved battery performance.
For example, what is called lithium ion polymer secondary batteries, wherein a solid electrolyte layer is disposed between a positive electrode and a negative electrode, have been drawing attention and being studied recently. The solid electrolyte layer is prepared such that a polymer substrate is impregnated with an electrolytic solution (lithium salt (electrolyte)+compatible solvent) which gels to show ion conductivity by itself. When a solid electrolyte layer is used, an electrolytic solution does not exist in a liquid state (the state where it flows by itself) within a battery, which in turn affords a huge advantage of absence of a leak from the battery. However, the battery characteristics (particularly, low temperature characteristics, cycle characteristics, high-rate discharge characteristics) of lithium ion secondary batteries using such a solid electrolyte layer tend to show inferior characteristics as compared to those of liquid type lithium ion secondary batteries (electrolytic solution+separator such as polyolefin), and the improvement thereof is a major goal.
In view of the above-mentioned situation, it is an object of the present invention to provide a lithium ion polymer secondary battery highly improved in all the low temperature characteristic, cycle characteristic and high-rate discharge characteristic.